9 Anetenna & Radio-Wave PropogationTroposphere :The lower part of the Earth’s atmosphere extendingUpwards from the Earth’s surface, in which temperaturedecreases with height except in local layers oftemperature inversion.This part of the atmosphere extends to an altitude ofabout 9 km at the Earth’s poles and 17 km at theequator.Stratosphere : 20 – 50 kmAnetenna & Radio-Wave Propogation

10 Anetenna & Radio-Wave PropogationIonosphere :That part of the upper atmosphere characterized by thepresence of ions and free electrons mainly arising fromphoto-ionization, the electron density being sufficient toproduce significant modification of the propagation of radiowaves in certain frequency bands.Note :The Earth’s ionosphere extends approximately from a heightof 50 km to a height of 2 000 km.Anetenna & Radio-Wave Propogation

14 Anetenna & Radio-Wave PropogationGround wave :A radio wave basically determined by the properties of theground which propagates in the troposphere and which is mainlydue to diffraction around the Earth.A ground wave is composed of two separate component waves-the surface wave and the space waveThe surface wave travels along the surface of the ground.A surface wave flows the curvature of the Earth due to theprocess of diffraction.The space wave follows two distinct paths from transmittingantenna to receiving antenna--one through the air directly to thereceiving antenna (direct wave or path), and the other reflectedfrom the ground to the receiving antenna (ground-reflected waveor path).Anetenna & Radio-Wave Propogation

18 Anetenna & Radio-Wave Propogationline-of-sight propagation :Propagation between two points for which the direct ray issufficiently clear of obstacles for diffraction to be of negligibleeffect.free-space propagation :Propagation of an electromagnetic wave in a homogeneousideal dielectric medium which may be considered of infiniteextent in all directions.Note : For propagation in free space, the magnitude of each vector of theelectromagnetic field in any given direction from the source beyond asuitable distance determined by the size of the source and the wavelengthis proportional to the reciprocal of the distance from the source.Anetenna & Radio-Wave Propogation

19 Anetenna & Radio-Wave PropogationImpedance of free space :The impedance of free space, Z0, is a physical constant relatingthe magnitudes of the electric and magnetic fields ofelectromagnetic radiation travelling through free space.That is, Z0 = |E|/|H|, where |E| is the electric field strength and|H| magnetic field strength.The impedance of free space equals the product of the vacuumpermeability Ɛ0 or magnetic constant μ0 and the speed of lightin a vacuum c; its value is approximately ohmsAnetenna & Radio-Wave Propogation

21 Anetenna & Radio-Wave PropogationFresnel zones :If unobstructed, radio waves will travel in a straight line fromthe transmitter to the receiver.But if there are obstacles near the path, the radio wavesreflecting off those objects may arrive out of phase with thesignals that travel directly and reduce the power of the receivedsignal.On the other hand, the reflection can enhance the power ofthe received signal if the reflection and the direct signals arrivein phase.Sometimes this results in the counterintuitive finding thatreducing the height of an antenna increases the signal-to-noiseratio.Anetenna & Radio-Wave Propogation

23 Fn = The nth Fresnel Zone radius in metres d1 = The distance of P from one end in metresd2 = The distance of P from the other end in metresλ = The wavelength of the transmitted signal in metresAnetenna & Radio-Wave Propogation